The present invention relates generally to the field of photometric detection and analysis, and more particularly to apparatus and methods for detecting and/or measuring the energy absorption by a fluid medium in a sample chamber.
In the qualitative and quantitative analysis of fluids, a well known analysis technique calls for a beam of electromagnetic radiation, such as visible light or infrared radiation, to be time-chopped (on-off modulated) to form a sample and a reference beam. The sample beam is directed through a sample of a fluid medium being examined; and selected components within the medium, if present, attentuate the energy of the incident beam at particular wavelengths characteristic of the selected fluid components. By measuring the amplitude of the sample beam, relative to the reference beam, after it passes through the fluid (making the measurement at one or more wavelengths), it is possible to both detect and identify the selected fluid components and determine the density thereof.
Generally, however, these photometric analysis measurement techniques attempt to measure small changes of signal which "ride" on a relatively large quiescent or base signal. Thus, changes of for example 0.1% or less in the overall signal are not unusual in order to accurately determine the quantity of selected fluids in the sample chamber. These small signal amplitude changes occur because the amount of energy absorption at a wavelength of interest is relatively small compared to the magnitude of the incident and measured signals. Therefore, in order to make these measurements extremely precise, low noise, complex measurement techniques are generally employed.
As a result, at least one known apparatus configuration mechanically modulates, by successively blocking and unblocking, a reference and a sample beam respectively, for providing two correlated modulated waveform beams having a 180.degree. out of phase AC component. These two beams, if not attenuated or if attenuated by the same amounts, can be combined to provide a single measurement signal having a constant amplitude. If however the beams are attenuated by different amounts, the combined measurement signal will have an AC component which can be measured and which is proportional to the difference in the amount of attenuation in the two amplitude modulated signals. This apparatus improves the accuracy of the absorption measurement in a gas analysis system but suffers from the requirement of precision mechanical modulation of separate energy signals and the difficulties concommittant therewith.
In addition, prior art devices generally use a single interrogation wavelength during the course of an investigation. To make measurement determinations at a plurality of wavelengths, it is generally necessary to replace either the energy source or the monochromatic filter being used, and to recalibrate the system accordingly. This procedure can prove both time consuming and inefficient.
In my application Ser. No. 178,190, referred to above, an apparatus and method for providing substantially improved sensitivity and reliability for photometric detection in fluids are described. In that apparatus, as well as in other dual component beam systems, as the sensitivity improves and approaches significantly low values, the apparatus is subject to inaccuracies due to the small effects such as for example, a change in energy source output, or different transmission or reflectance of various energy components due to changes of temperature, humidity, use, or other time dependent effects. These changes are not predictable and cannot be compensated for during the initial set up and calibration of the equipment. In addition, the erroneous results which can occur as a result of these time dependent changes in the relative signal amplitudes can provide both misleading and inaccurate results.
It is therefore a principal object of this invention to provide a detection and/or analysis apparatus and method with further improved precision by automatic compensation for changes as a result of time dependent mechanisms. Other objects of the invention are a fluid detection and/or analysis apparatus and method with reduced complexity, high efficiency, high reliability, and a relatively inexpensive manufacturing cost. A further object of the invention is a fluid detection and/or analysis apparatus and method which automatically compensate for relatively slow time varying changes in the component beams wherein the value of the time varying component is not predictable over time.